U.S. Pat. No. 3,469,982 discloses a photoresist process comprising applying a photopolymerizable layer supported on a support film to a surface, followed by imagewise exposure of the layer, stripping away the support film, and washing away the unexposed area of the layer to leave a resist image protecting the aforesaid surface from such treatments as etching or plating.
The patent discloses solution coating of a photopolymerizable composition onto the support film, followed by drying and then lamination of the resultant dry coating to a surface such as copper-clad printed circuit board (Example V) to carry out the photoresist process. In commercial practice, however, the dried photopolymerizable coating, which must remain adhered to the support during storage and shipment, has always been supplied sandwiched between the support film and a cover sheet, such as disclosed in Example I of the patent. This has enabled the sandwich to be rolled up upon itself by the photoresist manufacturer and be supplied to the user, e.g. printed circuit manufacturer, as a compact, easy to handle roll. The cover sheet permitted the roll to be unrolled by the user. Without the cover film, pressing the photopolymerizable layer against the backside of the support film would cause that layer to adhere. Consequently, the film could not be unrolled without damaging the photopolymerizable layer. In use, the cover sheet would be stripped away and discarded, followed by the photoresist processing described above. U.S. Pat. No. 3,782,939 discloses a need for the cover sheet because of the alleged tackiness of the photopolymerizable layer. In fact, the layer need not be tacky to the touch, but can adhere under pressure, for example when it is rolled up.
The equipment used to practice the photoresist process has consisted in general of separate pieces of equipment, such as a cleaner, scrubber for cleaning the copper-clad circuit boards, oven for preheating the boards, roll laminator applying heat to the photoresist and board as it laminates them together, and exposure (actinic radiation) station, and solvent development apparatus. Typically, manual transfer and positioning of the circuit board was necessary between each piece of equipment, involving exposure and less-than-desired reproducibility leading to yield loss in the process.
Various attempts have been made to automate the photoresist process but automation has been adopted to only a limited extent.
U.S. Pat. No. 3,547,730 discloses a process and apparatus for automating the photoresist process, involving a device for sequential feeding of shaped articles such as circuit boards and a laminator for receiving the boards and laminating a photosensitive layer thereto. Sequentially laminated boards become tied together by a web of support film and the photosensitive layer extending between boards as shown in FIG. 2, and the layer of the top and bottom web become laminated together to form a "hinge" (col. 5, lines 35-39). Next the support film is stripped away by rolls 12 and 13. This stripping away provides a forwarding motion to the circuit board to bring it into abutting relationship with the preceding board to force it into the exposure unit. The web of photosensitive layer left extending between successive boards after the support film is stripped away, folds over upon itself in the manner of a "hinge", when the boards become butted together. This hinge can be trimmed away by a knife prior to entering the exposure unit.
U.S. Pat. No. 3,547,730 also discloses the cleaning of a circuit board between a feed station and lamination station, the preheating of the board and/or photosensitive layer between their respective feeds and the lamination station, and rolls 10 for stripping away the cover sheet prior to lamination.
U.S. Pat. No. 4,025,380 discloses the sequential feeding of circuit boards onto a conveyor which passes them through an infrared preheater and then past sensor switches which control the operation of laminating rolls and cutting devices which cut a photosensitive layer to the desired length prior to lamination and then laminate the cut sheet (layer) to the circuit board. The patent also discloses rolls 63 for stripping off of the cover film prior to the lamination.
Improvements in specific process steps have been disclosed. U.S. Pat. No. 3,629,036, discloses that the elevated temperature required for laminating a photoresist to a substrate introduces disadvantages of limiting the types of photoresist and substrate materials that can be used and produces inferior bonding to the substrate. The patent purports to solve these problems by room temperature lamination after first applying a solution of dissolved resist to the substrate and then doing the lamination. The patent provides for a release layer between the photoresist layer and its support film to enable the film to be stripped away from the photoresist layer after lamination.
U.S. Pat. No. 3,794,546 discloses the use of an adhesive web to adhere to a support film of a photohardenable element so as to delaminate the support film from at least the unhardened area of a an imagewise exposed photohardenable layer laminated to a receptor sheet.
U.S. Pat. No. 4,075,051 discloses a process for self trimming of a photoresist layer laminated to a substrate, i.e., the portion of the layer extending past the edges of the substrate is removed without any cutting step. This is accomplished by applying a liquid weakening agent at least to the aforesaid portion of the layer, but possibly to the entire layer by pre-wetting of the substrate with the agent, and then peeling away either the cover sheet (support film) or an auxiliary permeable sheet which has a greater adhesion to the layer than the cohesive strength (as a result of the weakening agent) of the layer. The portion of the layer extending past the edge of the substrate tears along the edge of the substrate by the action of peeling away the cover sheet or permeable sheet and is removed with the sheet. The weakening agent is either a solvent for the photoresist or is a softening agent for it. The patent discloses the use of a conventional roll type laminator and pressure and heat for adhering the photoresist layer to the substrate.
Though the prior art methods of lamination are satisfactory for many applications, there is a need for even more precise and uniform lamination methods, especially where printed circuit boards having a very high line density are being fabricated.